1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy electrode capable of reversibly carrying out the electrochemical charge/discharge of hydrogen which is used as a negative electrode of a nickel-metal hydride battery, and more particularly to surface-treatment for activating the particle surface of the hydrogen-absorbing alloy.
2. Description of the Related Art
Traditionally, an alkaline battery has been employed as various kinds of power sources. A small-sized battery thereof has been used for various kinds of portable electronic/communicating appliances and a large-sized battery thereof has been used for an application of industry. In such a kind of alkaline battery, in most cases, a positive electrode is made of nickel compound. On the other hand, a negative electrode has been made of zinc, iron, hydrogen compound instead of cadmium compound, but mainly made of cadmium compound.
In recent years, in order to realize the alkaline battery with high energy density, the nickel-metal hydride battery containing a hydrogen-absorbing alloy electrode has attracted attention and has been put into practice. It has been known hydrogen-absorbing alloys using Tixe2x80x94Ni alloy, La (or Mm)xe2x80x94Ni alloy, etc.
The hydrogen-absorbing alloy used in the hydrogen-absorbing alloy electrode is a ground alloy which is obtained by grinding an alloy ingot, flake or spherical particle mechanically, or particle of a spherical or similar shape (elliptical) particle which is made by an atomization, a rotary disk method, a rotary nozzle method, etc.
Meanwhile, the surface of the hydrogen-absorbing alloy particle is very active. Therefore, when it exposed to air just a little, it immediately reacts with oxygen in the air. As a result, the alloy surface is oxidized to form an oxide layer. The oxide layer deteriorates the surface activity of the alloy, and particularly causes to reduction of the initial discharging capacity of the battery. Therefore, after the battery has been assembled, it must be subjected to charging/discharging in several cycles to several tens cycles to remove the oxide layer and activate the particle surface, thereby realizing the desired discharging capacity. This requires a large amount of labor and time.
In order to overcome such difficulty, it was proposed in JP-A-3-152868 that after the hydrogen-absorbing alloy particle has been treated by an acid solution, it is treated by an alkaline solution. In the technique of treatment proposed in JP-A-3-152868, since the hydrogen-absorbing alloy particle is treated by the alkaline solution after the acid treatment, its surface is covered with a porous layer mainly containing hydroxide. For this reason, even when the hydrogen-absorbing alloy is thereafter exposed to air, it will not be covered with a dense oxide layer so that the electrochemical activity of the surface will not be reduced.
However, in the method of treatment proposed in JP-A-3-152868, since the hydrogen-absorbing alloy particle is treated by the alkaline solution at room temperature after the acid treatment, metal ions dissolved in the alkaline solution cannot be sufficiently removed. The dissolved metal ions may be precipitated on the surface of the hydrogen-absorbing alloy again as hydroxide. This led to the problem of deterioration of the high rate discharging characteristic and low temperature discharging characteristic.
This invention has been accomplished in order to solve the above problem, and an object of this invention is to prevent precipitation of metal ions dissolved in a surface treatment solution to provide a hydrogen-absorbing alloy electrode which is excellent in the high rate discharging characteristic and low temperature discharging characteristic.
In order to attain the above object, a method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises the steps of:
dissolving a particle surface of said hydrogen-absorbing alloy by a surface-treatment solution; and
washing the hydrogen-absorbing alloy with the particle surface dissolved in an alkaline solution at a temperature not more than 40xc2x0 C., preferably at a temperature of 30xc2x0 C.xcx9c40xc2x0 C.
In this way, if the particle surface of said hydrogen-absorbing alloy, after having been dissolved by a surface-treatment solution is washed using an alkaline solution at a temperature of 30xc2x0 C.xcx9c40xc2x0 C., the metal ions dissolved by the surface-treatment solution can be completely washed away by the alkaline solution. Thus, the metal ions will not be precipitated onto the surface of the hydrogen-absorbing alloy again as a hydroxide. Particularly, in a nickel-metal hydride battery in which the alkaline solution is used as an electrolyte, washing by the alkaline solution prevents the metal ions from being dissolved within the battery. This improves the high rate discharging characteristic and the low temperature discharging characteristic of the hydrogen-absorbing alloy electrode.
Incidentally, the washing using the alkaline solution of which the temperature exceeds 40xc2x0 C. precipitates an alloy component as well as the hydroxide. Therefore, the temperature of the alkaline solution used must be controlled at 40xc2x0 C. or lower. On the other hand, if the temperature of the alkaline solution is lower than 30xc2x0 C., the washing effect cannot be sufficiently shown, therefore its temperature must be controlled at 30xc2x0 C. or higher.
The method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises:
dissolving a particle surface of said hydrogen-absorbing alloy by a surface-treatment solution; and
water-rinsing the hydrogen-absorbing alloy with the particle surface dissolved; and
washing the hydrogen-absorbing alloy thus water-rinsed in an alkaline solution at a temperature of 40xc2x0 C. or lower.
Most of the components solved by the surface-treatment solution are precipitated as hydroxides in an alkaline region. Therefore, if a hydrogen-absorbing alloy is treated with the alkaline solution, the hydroxide may be precipitated owing to changes in pH, therefore the washing effect cannot be shown sufficiently. In order to obviate such inconvenience, the hydrogen-absorbing alloy is water-rinsed after its surface is dissolved by the surface-treatment solution so that the dissolved element can be removed. Thus, the effect of the subsequent washing using the alkaline solution can be shown sufficiently. In this case, since the dissolved element has been already washed way, the temperature of the alkaline solution may be lower than 30xc2x0 C. as long as it is 40xc2x0 C. or lower.
The method of manufacturing a hydrogen-absorbing alloy electrode according to this invention comprises the steps of:
dissolving a particle surface of said hydrogen-absorbing alloy by a surface-treatment solution; and
treating the hydrogen-absorbing alloy with the particle surface dissolved using an acid solution of pH 4 or lower; and
washing the hydrogen-absorbing alloy thus treated using an alkaline solution at a temperature of 40xc2x0 C. or lower.
Most of the elements dissolved by the surface-treatment solution are precipitated as hydroxide in an alkaline region. Therefore, if the hydrogen-absorbing alloy is initially treated by a weak acid solution of pH 4 or lower, the hydroxide is not almost precipitated. This permits the washing to be carried out is a state where precipitation of the metal hydroxide has been suppressed. For this reason, the effect of the subsequent washing using the alkaline solution can be shown sufficiently. In this case also, since the dissolved elements have been already washed way, the temperature of the alkaline solution may be lower than 30xc2x0 C. as long as it is 40xc2x0 C. or lower.
Since treatment of the surface of the hydrogen-absorbing alloy by the surface-treatment solution intends to show the effects of increasing the surface area of the hydrogen-absorbing alloy and converting the surface of the hydrogen-absorbing alloy into a transient metal enriched layer, the surface-treatment solution is preferably an alkaline solution at a temperature of 60xc2x0 C. or higher, or an acid solution of pH3 or lower.